PI3Kδ activates E2F1 synthesis in response to mRNA translation stress

Sorafenib-associated translation reprogramming in hepatocellular carcinoma cells

Sorafenib (Sfb) is a multikinase inhibitor regularly used for the management of patients with advanced hepatocellular carcinoma (HCC) that has been shown to increase very modestly life expectancy. We have shown that Sfb inhibits protein synthesis at the level of initiation in cancer cells. However, the global snapshot of mRNA translation following Sorafenib-treatment has not been explored so far. In this study, we performed a genome-wide polysome profiling analysis in Sfb-treated HCC cells and demonstrated that, despite global translation repression, a set of different genes remain efficiently translated or are even translationally induced. We reveal that, in response to Sfb inhibition, translation is tuned, which strongly correlates with the presence of established mRNA cis-acting elements and the corresponding protein factors that recognize them, including DAP5 and ARE-binding proteins. At the level of biological processes, Sfb leads to the translational down-regulation of key cellular activities, such as those related to the mitochondrial metabolism and the collagen synthesis, and the translational up-regulation of pathways associated with the adaptation and survival of cells in response to the Sfb-induced stress. Our findings indicate that Sfb induces an adaptive reprogramming of translation and provides valuable information that can facilitate the analysis of other drugs for the development of novel combined treatment strategies based on Sfb therapy.

STAT3, MYC, and EBNA1 cooperate through a ZC3H18 transcriptional network to regulate survival and proliferation of EBV-positive lymphomas

Epstein-Barr virus (EBV), a common gamma-herpesvirus linked to various malignancies, exploits host cellular mechanisms to promote oncogenesis. Our previous research identified the zinc finger protein ZC3H18 as a novel component of the cellular DNA replication machinery in the context of EBV-driven tumorigenesis. We now demonstrate that ZC3H18 expression is upregulated in EBV-transformed and cancer cell lines, as well as in EBV-positive diffuse large B-cell lymphomas from AIDS patients, compared to their EBV-negative counterparts, supporting its activation by EBV. Our experiments show that ZC3H18 expression is regulated by the key oncogenic factors STAT3 and MYC, as well as the essential viral protein EBNA1. Using inhibitors and genetic knockdown, we find that suppressing STAT3, MYC, or EBNA1 leads to decreased ZC3H18 levels, reduced cell viability, and increased apoptosis in EBV-positive B lymphoma cells. Furthermore, ZC3H18, STAT3, MYC, and EBNA1 mutually support each other's expression through a complex transcriptional network. Notably also, ZC3H18 transcriptionally enhances components of the NF-κB pathway, contributing to NF-κB signaling even in the absence of the EBV oncoprotein LMP1, which is crucial for cell proliferation and survival of several EBV-associated malignancies. Our findings reveal a novel regulatory axis in EBV-positive cancer cells involving STAT3, MYC, EBNA1, & ZC3H18, also linking ZC3H18 to the NF-κB pathway independently of LMP1. The involvement of EBNA1 in this network may explain, at least in part, the preferential upregulation of ZC3H18 in EBV-associated tumors and highlights predictive and therapeutic possibilities for such cancers.

Translation of bi-directional transcripts enhances MHC-I peptide diversity

Antisense transcripts play an important role in generating regulatory non-coding RNAs but whether these transcripts are also translated to generate functional peptides remains poorly understood. In this study, RNA sequencing and six-frame database generation were combined with mass spectrometry analysis of peptides isolated from polysomes to identify Nascent Pioneer Translation Products (Na-PTPs) originating from alternative reading frames of bi-directional transcripts. Two Na-PTP originating peptides derived from antisense strands stimulated CD8+ T cell proliferation when presented to peripheral blood mononuclear cells (PBMCs) from nine healthy donors. Importantly, an antigenic peptide derived from the reverse strand of two cDNA constructs was presented on MHC-I molecules and induced CD8+ T cell activation. The results demonstrate that three-frame translation of bi-directional transcripts generates antigenic peptide substrates for the immune system. This discovery holds significance for understanding the origin of self-discriminating peptide substrates for the major histocompatibility class I (MHC-I) pathway and for enhancing immune-based therapies against infected or transformed cells.

Pre-Defined Stem-Loop Structure Library for the Discovery of L-RNA Aptamers that Target RNA G-Quadruplexes

L-RNA aptamers have been developed to target G-quadruplexes (G4s) and regulate G4-mediated gene expression. However, the aptamer selection process is laborious and challenging, and aptamer identification is subject to high failure rates. By analyzing the previously reported G4-binding L-RNA aptamers, we found that the stem-loop (SL) structure is favored by G4 binding. Herein, we present a robust and effective G4-SLSELEX-Seq platform specifically for G4 targets by introducing a pre-defined stem-loop structure library during the SELEX process. Using G4-SLSELEX-Seq, we identified an L-RNA aptamer, L-Apt1-12, for the Epstein-Barr nuclear antigen 1 (EBNA1) RNA G4 (rG4) in just three selection rounds. L-Apt1-12 maintained the stem-loop structure initially introduced, and possessed a unique G-triplex motif that is important for the strong binding affinity and specificity to EBNA1 rG4. L-Apt1-12 effectively downregulated endogenous EBNA1 protein expression in human cancer cells and showed selective toxicity towards Epstein-Barr virus (EBV)-positive cancer cells, highlighting its potential for targeted therapy against EBV-associated cancers. Furthermore, we demonstrated the robustness and generality of G4-SLSELEX-Seq by selecting L-RNA aptamers for the amyloid precursor protein (APP) rG4 and the hepatitis C virus subtype 1a (HCV-1a) rG4, obtaining high-affinity aptamers in three selection rounds. These findings demonstrated G4-SLSELEX-Seq as a robust and efficient platform for the selection of rG4-targeting L-RNA aptamers.

Tumour heterogeneity and personalized treatment screening based on single-cell transcriptomics

According to global cancer statistics for the year 2022, based on updated estimates from the International Agency for Research on Cancer, there were approximately 20 million new cases of cancer in 2022 alongside 9.7 million related deaths. Lung, breast, colorectal, gastric, and liver cancers are the most common types of cancer. Despite advancements in anticancer drugs and optimised chemotherapy regimens that have improved cure rates for malignant tumours, the presence of tumour heterogeneity has resulted in substantial variations among patients in terms of disease progression, clinical response, sensitivity to therapy, and prognosis, posing significant challenges in attaining optimal therapeutic outcomes for each patient. Here, we collected five single-cell transcriptome datasets from patients with lung, breast, colorectal, gastric, and liver cancers and constructed multiple cancer blueprints of tumour cell heterogeneity. By integrating multiple bioinformatics analyses, we explored the biological differences underlying tumour cell heterogeneity at the single-cell level and identified tumour cell subcluster-specific biomarkers and potential therapeutic drugs for each subcluster. Interestingly, although tumour cell subpopulations exhibit dramatic differences within the same cancer type and between different cancers at both the genomic and transcriptomic levels, some demonstrate similar oncogenic pathway activities and phenotypes. Tumour cell subpopulations from the five cancers listed above were classified into three major groups corresponding to different treatment strategies. The findings of this study not only focus on the differences but also on the similarities among tumour cell subpopulations across different cancers, providing new insights for individualised therapy.

The hide-and-seek game of the oncogenic Epstein-Barr virus-encoded EBNA1 protein with the immune system: An RNA G-quadruplex tale

The Epstein-Barr virus (EBV) is the first oncogenic virus described in human. EBV infects more than 90% of the human population worldwide, but most EBV infections are asymptomatic. After the primary infection, the virus persists lifelong in the memory B cells of the infected individuals. Under certain conditions the virus can cause several human cancers, that include lymphoproliferative disorders such as Burkitt and Hodgkin lymphomas and non-lymphoid malignancies such as 100% of nasopharyngeal carcinoma and 10% of gastric cancers. Each year, about 200,000 EBV-related cancers emerge, hence accounting for at least 1% of worldwide cancers. Like all gammaherpesviruses, EBV has evolved a strategy to escape the host immune system. This strategy is mainly based on the tight control of the expression of its Epstein-Barr nuclear antigen-1 (EBNA1) protein, the EBV-encoded genome maintenance protein. Indeed, EBNA1 is essential for viral genome replication and maintenance but, at the same time, is also highly antigenic and T cells raised against EBNA1 exist in infected individuals. For this reason, EBNA1 is considered as the Achilles heel of EBV and the virus has seemingly evolved a strategy that employs the binding of nucleolin, a host cell factor, to RNA G-quadruplex (rG4) within EBNA1 mRNA to limit its expression to the minimal level required for function while minimizing immune recognition. This review recapitulates in a historical way the knowledge accumulated on EBNA1 immune evasion and discusses how this rG4-dependent mechanism can be exploited as an intervention point to unveil EBV-related cancers to the immune system.

The p53 endoplasmic reticulum stress-response pathway evolved in humans but not in mice via PERK-regulated p53 mRNA structures

Cellular stress conditions activate p53-dependent pathways to counteract the inflicted damage. To achieve the required functional diversity, p53 is subjected to numerous post-translational modifications and the expression of isoforms. Little is yet known how p53 has evolved to respond to different stress pathways. The p53 isoform p53/47 (p47 or ΔNp53) is linked to aging and neural degeneration and is expressed in human cells via an alternative cap-independent translation initiation from the 2nd in-frame AUG at codon 40 (+118) during endoplasmic reticulum (ER) stress. Despite an AUG codon in the same location, the mouse p53 mRNA does not express the corresponding isoform in either human or mouse-derived cells. High-throughput in-cell RNA structure probing shows that p47 expression is attributed to PERK kinase-dependent structural alterations in the human p53 mRNA, independently of eIF2α. These structural changes do not take place in murine p53 mRNA. Surprisingly, PERK response elements required for the p47 expression are located downstream of the 2nd AUG. The data show that the human p53 mRNA has evolved to respond to PERK-mediated regulation of mRNA structures in order to control p47 expression. The findings highlight how p53 mRNA co-evolved with the function of the encoded protein to specify p53-activities under different cellular conditions.

Convallatoxin Inhibits Cell Proliferation and Induces Cell Apoptosis by Attenuating the Akt-E2F1 Signaling Pathway in K562 Cells

Objective: To determine the effect of convallatoxin on K562 cell proliferation and apoptosis. Methods: CCK-8 assay was used to detect cell proliferation; PI staining, JC-1 staining, and Annexin V-FITC/PI double staining were used to analyze the cell cycle, cell mitochondrial membrane potential, and cell apoptosis; and Western blotting was used to detect cleaved caspase-9, cleaved caspase-3, Bcl-2, Bax, and E2F1 expression and Akt phosphorylation. Subsequently, AutoDock software was used to determine the interaction between convallatoxin and Akt1. Results: Upon treatment with convallatoxin, the proliferation of K562 cells was inhibited, the cells were arrested at the S and G2/M phases, and cell apoptosis was significantly induced. In addition, Akt phosphorylation and E2F1 expression were significantly decreased, whereas E2F1 overexpression rescued convallatoxin-induced cell proliferation and apoptosis. In addition, a molecular docking assay indicated that convallatoxin could bind to Akt1. Conclusion: Convallatoxin inhibited cell proliferation and induced mitochondrial-related apoptosis in K562 cells by reducing the Akt-E2F1 signaling pathway, indicating that it is a potential agent for treating leukemia.

Ribosomopathies and cancer: pharmacological implications

INTRODUCTION

The ribosome is a ribonucleoprotein organelle responsible for protein synthesis, and its biogenesis is a highly coordinated process that involves many macromolecular components. Any acquired or inherited impairment in ribosome biogenesis or ribosomopathies is associated with the development of different cancers and rare genetic diseases. Interference with multiple steps of protein synthesis has been shown to promote tumor cell death.

AREAS COVERED

We discuss the current insights about impaired ribosome biogenesis and their secondary consequences on protein synthesis, transcriptional and translational responses, proteotoxic stress, and other metabolic pathways associated with cancer and rare diseases. Studies investigating the modulation of different therapeutic chemical entities targeting cancer in in vitro and in vivo models have also been detailed.

EXPERT OPINION

Despite the association between inherited mutations affecting ribosome biogenesis and cancer biology, the development of therapeutics targeting the essential cellular machinery has only started to emerge. New chemical entities should be designed to modulate different checkpoints (translating oncoproteins, dysregulation of specific ribosome-assembly machinery, ribosomal stress, and rewiring ribosomal functions). Although safe and effective therapies are lacking, consideration should also be given to using existing drugs alone or in combination for long-term safety, with known risks for feasibility in clinical trials and synergistic effects.

Activation of DNA methyltransferase 3a by Epstein-Barr nuclear antigen 1 in gastric carcinoma

BACKGROUND

Epstein-Barr nuclear antigen 1 (EBNA1) is expressed in all Epstein-Barr virus (EBV)-infected cells. It interacts with a variety of cellular proteins and activates the transcription of other EBV latency genes, which plays an important role in the persistence of the EBV genome during latent infection.

AIM

Several studies have shown that EBV infection induces the expression of DNA methyltransferases (DNMTs) and causes extensive methylation of the whole genome in EBV-associated gastric carcinoma (EBVaGC). However, the specific mechanism by which EBV regulates DNMTs expression is still unclear.

METHODS AND RESULTS

EBNA1 plasmid and siRNA were transfected to evaluate the effect of EBNA1 on DNMT3a expression. Molecular biology experiments were used to detect the biological function of DNMT3a and its effect on EBV latency in gastric carcinoma cells. We showed that EBNA1 upregulated DNMT3a expression through the E2F1 transcription factor (E2F1) in EBVaGC. DNMT3a knockdown restrained cell proliferation, induced cell cycle arrest, promoted cell apoptosis and suppressed cell migration in vitro.

CONCLUSIONS

Our results showed a new mechanism for EBV to regulate the expression of DNMT3a. Targeting the EBNA1/E2F1/DNMT3a axis may provide an alternative therapeutic strategy in the treatment of EBVaGC with high DNMT3a expression.

The different activities of RNA G-quadruplex structures are controlled by flanking sequences

The role of G-quadruplex (G4) RNA structures is multifaceted and controversial. Here, we have used as a model the EBV-encoded EBNA1 and the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded LANA1 mRNAs. We have compared the G4s in these two messages in terms of nucleolin binding, nuclear mRNA retention, and mRNA translation inhibition and their effects on immune evasion. The G4s in the EBNA1 message are clustered in one repeat sequence and the G4 ligand PhenDH2 prevents all G4-associated activities. The RNA G4s in the LANA1 message take part in similar multiple mRNA functions but are spread throughout the message. The different G4 activities depend on flanking coding and non-coding sequences and, interestingly, can be separated individually. Together, the results illustrate the multifunctional, dynamic and context-dependent nature of G4 RNAs and highlight the possibility to develop ligands targeting specific RNA G4 functions. The data also suggest a common multifunctional repertoire of viral G4 RNA activities for immune evasion.

The Epstein-Barr virus noncoding RNA EBER2 transactivates the UCHL1 deubiquitinase to accelerate cell growth

The Epstein-Barr virus (EBV) transforms resting B cells and is involved in the development of B cell lymphomas. We report here that the viral noncoding RNA EBER2 accelerates B cell growth by potentiating expression of the UCHL1 deubiquitinase that itself increased expression of the Aurora kinases and of cyclin B1. Importantly, this effect was also visible in Burkitt's lymphoma cells that express none of the virus's known oncogenes. Mechanistically, EBER2 bound the UCHL1 messenger RNA (mRNA), thereby bringing a protein complex that includes PU.1, a UCHL1 transactivator, to the vicinity of its promoter. Although the EBV oncogene LMP1 has been suggested to induce UCHL1, we show here that EBER2 plays a much more important role to reach significant levels of the deubiquitinase in infected cells. However, some viruses that carried a polymorphic LMP1 had an increased ability to achieve full UCHL1 expression. This work identifies a direct cellular target of a viral noncoding RNA that is likely to be central to EBV's oncogenic properties.

p53 mRNA Metabolism Links with the DNA Damage Response

Human cells are subjected to continuous challenges by different genotoxic stress attacks. DNA damage leads to erroneous mutations, which can alter the function of oncogenes or tumor suppressors, resulting in cancer development. To circumvent this, cells activate the DNA damage response (DDR), which mainly involves cell cycle regulation and DNA repair processes. The tumor suppressor p53 plays a pivotal role in the DDR by halting the cell cycle and facilitating the DNA repair processes. Various pathways and factors participating in the detection and repair of DNA have been described, including scores of RNA-binding proteins (RBPs) and RNAs. It has become increasingly clear that p53's role is multitasking, and p53 mRNA regulation plays a prominent part in the DDR. This review is aimed at covering the p53 RNA metabolism linked to the DDR and highlights the recent findings.

Viruses, cancer and non-self recognition

Virus-host interactions form an essential part of every aspect of life, and this review is aimed at looking at the balance between the host and persistent viruses with a focus on the immune system. The virus-host interaction is like a cat-and-mouse game and viruses have developed ingenious mechanisms to manipulate cellular pathways, most notably the major histocompatibility (MHC) class I pathway, to reside within infected cell while evading detection and destruction by the immune system. However, some of the signals sensing and responding to viral infection are derived from viruses and the fact that certain viruses can prevent the infection of others, highlights a more complex coexistence between the host and the viral microbiota. Viral immune evasion strategies also illustrate that processes whereby cells detect and present non-self genetic material to the immune system are interlinked with other cellular pathways. Immune evasion is a target also for cancer cells and a more detailed look at the interfaces between viral factors and components of the MHC class I peptide-loading complex indicates that these interfaces are also targets for cancer mutations. In terms of the immune checkpoint, however, viral and cancer strategies appear different.

Immunology of EBV-Related Lymphoproliferative Disease in HIV-Positive Individuals

Epstein-Bar virus (EBV) can directly cause lymphoproliferative disease (LPD), including AIDS-defining lymphomas such as Burkitt’s lymphoma and other non-Hodgkin lymphomas (NHL), as well as human immunodeficiency virus (HIV)-related Hodgkin lymphoma (HL). The prevalence of EBV in HL and NHL is elevated in HIV-positive individuals compared with the general population. Rates of incidence of AIDS-defining cancers have been declining in HIV-infected individuals since initiation of combination anti-retroviral therapy (cART) use in 1996. However, HIV-infected persons remain at an increased risk of cancers related to infections with oncogenic viruses. Proposed pathogenic mechanisms of HIV-related cancers include decreased immune surveillance, decreased ability to suppress infection-related oncogenic processes and a state of chronic inflammation marked by alteration of the cytokine profile and expanded numbers of cytotoxic T lymphocytes with down-regulated co-stimulatory molecules and increased expression of markers of senescence in the setting of treated HIV infection. Here we discuss the cooperation of EBV-infected B cell- and environment-associated factors that may contribute to EBV-related lymphomagenesis in HIV-infected individuals. Environment-derived lymphomagenic factors include impaired host adaptive and innate immune surveillance, cytokine dysregulation and a pro-inflammatory state observed in the setting of chronic, cART-treated HIV infection. B cell factors include distinctive EBV latency patterns and host protein expression in HIV-associated LPD, as well as B cell-stimulating factors derived from HIV infection. We review the future directions for expanding therapeutic approaches in targeting the viral and immune components of EBV LPD pathogenesis.

MDM2's dual mRNA binding domains co-ordinate its oncogenic and tumour suppressor activities

Cell growth requires a high level of protein synthesis and oncogenic pathways stimulate cell proliferation and ribosome biogenesis. Less is known about how cells respond to dysfunctional mRNA translation and how this feeds back into growth regulatory pathways. The Epstein-Barr virus (EBV)-encoded EBNA1 causes mRNA translation stress in cis that activates PI3Kδ. This leads to the stabilization of MDM2, induces MDM2's binding to the E2F1 mRNA and promotes E2F1 translation. The MDM2 serine 166 regulates the interaction with the E2F1 mRNA and deletion of MDM2 C-terminal RING domain results in a constitutive E2F1 mRNA binding. Phosphorylation on serine 395 following DNA damage instead regulates p53 mRNA binding to its RING domain and prevents the E2F1 mRNA interaction. The p14Arf tumour suppressor binds MDM2 and in addition to preventing degradation of the p53 protein it also prevents the E2F1 mRNA interaction. The data illustrate how two MDM2 domains selectively bind specific mRNAs in response to cellular conditions to promote, or suppress, cell growth and how p14Arf coordinates MDM2's activity towards p53 and E2F1. The data also show how EBV via EBNA1-induced mRNA translation stress targets the E2F1 and the MDM2 - p53 pathway.

Phosphoinositide 3-kinase δ inactivation prevents vitreous-induced activation of AKT/MDM2/p53 and migration of retinal pigment epithelial cells

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that play a critical role in transmitting signals from cell-surface molecules to intracellular protein effectors. Key PI3Ks include PI3Kα, PI3Kβ, and PI3Kδ, which are regulated by receptors. The signaling pathway comprising the PI3Ks, along with a Ser/Thr kinase (AKT), a proto-oncogene product (mouse double minute (MDM)2), and a tumor suppressor protein (p53), plays an essential role in experimental proliferative vitreoretinopathy (PVR), which is a fibrotic blinding eye disorder. However, which PI3K isoforms are involved in PVR is unknown. A major characteristic of PVR is the formation of epi (or sub)-retinal membranes that consist of extracellular matrix and cells, including retinal pigment epithelium (RPE) cells, glial cells, and macrophages. RPE cells are considered key players in PVR pathogenesis. Using immunoblotting and immunofluorescence analyses, we herein provide the evidence that PI3Kδ is highly expressed in human RPEs when it is primarily expressed in leukocytes. We also found that PI3Kδ inactivation through two approaches, CRISPR/Cas9-mediated depletion and a PI3Kδ-specific inhibitor (idelalisib), not only blocks vitreous-induced activation of AKT and MDM2 but also abrogates a vitreous-stimulated decrease in p53. Furthermore, we demonstrate that PI3Kδ inactivation prevents vitreous-induced proliferation, migration, and contraction of human RPEs. These results suggest that PI3Kδ may represent a potential therapeutic target for RPE-related eye diseases, including PVR.

Mechanisms of B-Cell Oncogenesis Induced by Epstein-Barr Virus

Epstein-Barr virus (EBV) is a ubiquitous gammaherpesvirus which asymptomatically infects the majority of the world population. Under immunocompromised conditions, EBV can trigger human cancers of epithelial and lymphoid origin. The oncogenic potential of EBV is demonstrated by in vitro infection and transformation of quiescent B cells into lymphoblastoid cell lines (LCLs). These cell lines, along with primary infection using genetically engineered viral particles coupled with recent technological advancements, have elucidated the underlying mechanisms of EBV-induced B-cell lymphomagenesis.

Nuclear processing of nascent transcripts determines synthesis of full-length proteins and antigenic peptides

Peptides presented on major histocompatibility (MHC) class I molecules form an essential part of the immune system's capacity to detect virus-infected or transformed cells. Earlier works have shown that pioneer translation peptides (PTPs) for the MHC class I pathway are as efficiently produced from introns as from exons, or from mRNAs targeted for the nonsense-mediated decay pathway. The production of PTPs is a target for viral immune evasion but the underlying molecular mechanisms that govern this non-canonical translation are unknown. Here, we have used different approaches to show how events taking place on the nascent transcript control the synthesis of PTPs and full-length proteins. By controlling the subcellular interaction between the G-quadruplex structure (G4) of a gly-ala encoding mRNA and nucleolin (NCL) and by interfering with mRNA maturation using multiple approaches, we demonstrate that antigenic peptides derive from a nuclear non-canonical translation event that is independently regulated from the synthesis of full-length proteins. Moreover, we show that G4 are exploited to control mRNA localization and translation by distinguishable mechanisms that are targets for viral immune evasion.

Epstein–Barr Virus and Cancer

Epstein–Barr virus (EBV) contributes to about 1.5% of all cases of human cancer worldwide, and viral genes are expressed in the malignant cells. EBV also very efficiently causes the proliferation of infected human B lymphocytes. The functions of the viral proteins and small RNAs that may contribute to EBV-associated cancers are becoming increasingly clear, and a broader understanding of the sequence variation of the virus genome has helped to interpret their roles. The improved understanding of the mechanisms of these cancers means that there are great opportunities for the early diagnosis of treatable stages of EBV-associated cancers and the use of immunotherapy to target EBV-infected cells or overcome immune evasion. There is also scope for preventing disease by immunization and for developing therapeutic agents that target the EBV gene products expressed in the cancers.

Translation Stress Regulates Ribosome Synthesis and Cell Proliferation

Ribosome and protein synthesis are major metabolic events that control cellular growth and proliferation. Impairment in ribosome biogenesis pathways and mRNA translation is associated with pathologies such as cancer and developmental disorders. Processes that control global protein synthesis are tightly regulated at different levels by numerous factors and linked with multiple cellular signaling pathways. Several of these merge on the growth promoting factor c-Myc, which induces ribosome biogenesis by stimulating Pol I, Pol II, and Pol III transcription. However, how cells sense and respond to mRNA translation stress is not well understood. It was more recently shown that mRNA translation stress activates c-Myc, through a specific induction of E2F1 synthesis via a PI3Kδ-dependent pathway. This review focuses on how this novel feedback pathway stimulates cellular growth and proliferation pathways to synchronize protein synthesis with ribosome biogenesis. It also describes for the first time the oncogenic activity of the mRNA, and not the encoded protein.

Lymphomas driven by Epstein-Barr virus nuclear antigen-1 (EBNA1) are dependant upon Mdm2

Epstein-Barr virus (EBV) associated Burkitt’s lymphoma is characterised by the deregulation of c-Myc expression and a restricted viral gene expression pattern in which the EBV nuclear antigen-1 (EBNA1) is the only viral protein to be consistently expressed. EBNA1 is required for viral genome propagation and segregation during latency. However, it has been much debated whether the protein plays a role in viral-associated tumourigenesis. We show that the lymphomas which arise in EµEBNA1 transgenic mice are unequivocally linked to EBNA1 expression and that both C-Myc and Mdm2 deregulation are central to this process. Tumour cell survival is supported by IL-2 and there is a skew towards CD8 positive T-cells in the tumour environment, while the immune check-point protein PD-L1 is upregulated in the tumours. Additionally, several isoforms of Mdm2 are upregulated in the EµEBNA1 tumours, with increased phosphorylation at ser166, an expression pattern not seen in Eµc-Myc transgenic tumours. Concomitantly, E2F1, Xiap, Mta1, C-Fos and Stat1 are upregulated in the tumours. Using four independent inhibitors of Mdm2 we demonstrate that the EµEBNA1 tumour cells are dependant upon Mdm2 for survival (as they are upon c-Myc) and that Mdm2 inhibition is not accompanied by upregulation of p53, instead cell death is linked to loss of E2F1 expression, providing new insight into the underlying tumourigenic mechanism. This opens a new path to combat EBV-associated disease.

EBNA1: Oncogenic Activity, Immune Evasion and Biochemical Functions Provide Targets for Novel Therapeutic Strategies against Epstein-Barr Virus- Associated Cancers

The presence of the Epstein-Barr virus (EBV)-encoded nuclear antigen-1 (EBNA1) protein in all EBV-carrying tumours constitutes a marker that distinguishes the virus-associated cancer cells from normal cells and thereby offers opportunities for targeted therapeutic intervention. EBNA1 is essential for viral genome maintenance and also for controlling viral gene expression and without EBNA1, the virus cannot persist. EBNA1 itself has been linked to cell transformation but the underlying mechanism of its oncogenic activity has been unclear. However, recent data are starting to shed light on its growth-promoting pathways, suggesting that targeting EBNA1 can have a direct growth suppressing effect. In order to carry out its tasks, EBNA1 interacts with cellular factors and these interactions are potential therapeutic targets, where the aim would be to cripple the virus and thereby rid the tumour cells of any oncogenic activity related to the virus. Another strategy to target EBNA1 is to interfere with its expression. Controlling the rate of EBNA1 synthesis is critical for the virus to maintain a sufficient level to support viral functions, while at the same time, restricting expression is equally important to prevent the immune system from detecting and destroying EBNA1-positive cells. To achieve this balance EBNA1 has evolved a unique repeat sequence of glycines and alanines that controls its own rate of mRNA translation. As the underlying molecular mechanisms for how this repeat suppresses its own rate of synthesis in cis are starting to be better understood, new therapeutic strategies are emerging that aim to modulate the translation of the EBNA1 mRNA. If translation is induced, it could increase the amount of EBNA1-derived antigenic peptides that are presented to the major histocompatibility (MHC) class I pathway and thus, make EBV-carrying cancers better targets for the immune system. If translation is further suppressed, this would provide another means to cripple the virus.